The invention relates to an aluminum-magnesium alloy for casting operations and to the use of this aluminum-magnesium alloy in casting operations, in particular in die-casting operations. Further the invention relates to the application of the AlMg casting alloy in automotive components.
Conventional aluminum-magnesium casting alloys have many attractive properties, such as high ultimate tensile strength ( greater than 170 MPa) and elongation ( greater than 8%) with moderate yield strength ( greater than 120 MPa). However, there is a demand for aluminum cast alloys combining improved mechanical properties with a good corrosion resistance.
Some disclosures of aluminum-magnesium casting alloys found in the prior art is literature will be mentioned below.
WO-96/15281 discloses a casting alloy consisting of, in weight percent:
balance aluminum and impurities.
The alloy may be used in a die-casting operation, and appears to be particularly suitable for use in thixocasting and rheocasting operations.
WO-96/25528 discloses a casting alloy consisting of, in weight percent:
balance aluminum and impurities.
Optionally the alloy may further comprise 0.01-0.04% Ti and/or 0.01-0.10% Zn. The alloy can be employed in die-casting operations, the alloy is capable of having a yield strength greater than or equal to 110 MPa and an elongation greater than or equal to 17%.
WO-96/30554 discloses a casting alloy consisting of, in weight percent:
balance aluminum and impurities.
The alloy may be used in die-casting operations. The casting alloy is particularly suitable for manufacturing safety components for cars. Typical strength levels disclosed in a T5-temper are YS of 116 MPa and UTS of 219 MPa and elongation of 19%.
JP-A-09-003582 discloses an aluminum casting alloy having, in weight percent:
balance aluminum and inevitable impurities.
The cast products have in its matrix dispersed crystallised products in a spheroidising way.
It is an object of this invention to provide an aluminum-magnesium alloy that can be used in a variety of casting operations.
It is another object of this invention to provide an aluminum-magnesium alloy ideally suited for use in die-casting operations.
It is also an object of this invention to provide improved cast products and components consisting of an improved aluminum-magnesium alloy cast members that ideally are suited for automotive applications.
It is also an object of this invention to provide an aluminum-magnesium alloy having in the as-cast condition the following minimum mechanical properties: yield strength of at least 120 MPa, a tensile strength of at least 180 MPa and an elongation of at least 7%.
According to the invention there is provided an aluminum-magnesium casting alloy, having the following composition in weight percent:
balance aluminum and inevitable impurities.
By the invention cast products or cast bodies can be provided having higher strength in combination with higher elongation. In addition these products have a good corrosion resistance and can be welded using known welding techniques for this type of casting alloys. Alloys of the present invention have also been found with a good castability, in particular in die-casting operations, and no soldering occur when using the casting alloy. The aluminum casting alloy according to the invention is capable of achieving in the as-cast condition a yield strength of more than 140 MPa, in combination with a tensile strength of more than 200 MPa and an elongation at fracture of more than 7%.
The invention also includes products made from the aluminum casting alloy set out above. Typical examples of such products are die-cast, in particular high pressure die-cast, products such as safety components, vehicle wheels, steering wheels, steering columns, airbag modules/cans, brake drums and frame members for a vehicle. The alloy is particularly suited for any application having load and impact requirements where properties of high strength and high elongation are desirable. Typical safety components for cars include structural parts for crush zones or otherwise protecting car passengers. It is known to provide one or more crush zones in a frame of a vehicle such as an automobile. The crush zones are designed to crush, or deform, in the event of a vehicle collision. The deformation of the frame absorbs energy of the collision to help protect an occupant of the vehicle. The frame may be configured to deform in a certain manner upon the application of force exceeding a predetermined amount.
The present aluminum-magnesium alloy is environmentally friendly and is readily recyclable because it does not contaminate the wrought alloy stream of recycled materials. The alloy is typically solidified into ingot-derived stock by continuous casting or semi-continuous casting into a shape suitable for remelt for casting, which shape is typically an ingot billet.
It should be mentioned here that from WO-97/38146, incorporated herein by reference, an aluminum wrought alloy is known for application as rolled sheet or plate or as an extrusion, and having a composition, in weight percent:
balance Al and inevitable impurities.
The aluminum wrought alloy disclosed in this international patent application is also subject of the Aluminum Association registration number AA5069.
From WO-99-42627, incorporated herein by reference, an aluminum-magnesium alloy in the form of a rolled product or an extrusion is known, having the following composition, in weight percent:
balance Al and inevitable impurities.
However, neither in WO-97/38146, nor in WO-99/42627 it is mentioned or suggested that the aluminum wrought alloy could be successfully used as an aluminum casting alloy, in particular as an aluminum die-casting alloy.
It is believed that the improved properties available with the casting alloy of the invention, particularly higher strength levels and high elongation in combination with good casting characteristics, result from the combined additions of Mg, Mn and Zn in the given ranges. The aluminum casting alloy is therefore ideally suited for the improved post casting processing, i.e. the elimination of conventional high temperature solution heat treating and optionally ageing at room temperature or elevated temperature, while providing even complexly shaped die-cast products with improved dimensional stability and mechanical properties.
The reasons for the limitations of the alloying elements of the aluminum casting alloy according to the present invention are described below. All composition percentages are by weight.
Mg is the primary strengthening element in the alloy. Depending on the field of application, the magnesium content is preferably in the range of 2.7 to 6.0%. Mg levels below 2.7% do not provide the required strength and when the addition exceeds 6.0%, problems during casting occur. A more preferred minimum Mg level is 3.0%. The preferred level of Mg is 4.5 to 6.0%, more preferably 5.0 to 6.0%, and most preferably 5.2 to 5.8%, as a compromise between ease of casting, strength and corrosion resistance.
In another embodiment the Mg-level is in the range of 2.7 to 4.5%, and preferably 3.0 to 4.5%. In this range and in combination with the other alloying elements, the aluminum casting alloy is capable of obtaining in the as-cast condition a UTS of at least 210 MPa, a YS of at least 120 MPa, and an elongation of at least 17%, and in the best examples an elongation of 23% or more.
Mn is an essential additive element. In combination with Mg and Zn, Mn provides the strength in the as-cast condition and the welded joints of the alloy. Mn levels below 0.4% cannot provide sufficient strength and corrosion resistance to the alloy. Above 1.4% the castability becomes increasingly difficult. The preferred level of Mn is 0.45 to 1.2%, and more preferably 0.45 to 0.8%, which represents a balanced compromise between strength, corrosion resistance, and castability.
Zn is also an essential alloying element. In combination with Mg, Zn provides the strength in the as-cast condition and the welded joints of the alloy. Furthermore, the addition of Zn results in a good corrosion resistance of the aluminum cast alloy. Zn should be present in a range of 0.10 to 1.5%. At a level above 1.5% Zn the castability becomes increasingly difficult. A preferred range for Zn is 0.3 to 1.4%, more preferably 0.4 to 1.1%, and most preferably 0.45 to 0.9%, which represents a compromise between strength, corrosion resistance and castability. Usually Zn is considered in the art as an impurity element in AlMg casting alloys, which should be kept at a level as low as possible, preferably maximum of 0.10% and more preferably maximum of 0.05%. However, in the alloy according to the invention Zn may be present as an alloying element resulting in beneficial effects. Although not yet fully understood, it is believed that the addition of Zn contributes to the good casting characteristics of the alloy, such as a low tendency to die-sticking when used in a die-casting operation. Good results are being achieved in those examples having a Mg/Zn-ratio of 6.0 or more.
Zr is for achieving strength improvement in the cast product. Zr also improves the weldability of the cast product. Zr levels above 0.3% tend not to have any further advantages. The preferred level of Zr is in the range of 0.05 to 0.25%, and more preferably 0.06-0.16%.
V may be added for achieving further improvements in the mechanical properties of the cast product, in particular mechanical properties at elevated temperatures. If added, the preferred level of V is in the range of 0.05 to 0.25%, and more preferably in the range of 0.1 to 0.2%. The addition of V in the given range may in particular result in a further improved ductility of the alloy, in particular when heat treated following casting at a temperature in a range of 200 to 400xc2x0 C.
Sc may be added to the alloy for improving the weldability of a cast product. The Sc may be added alone or in combination with Zr in a range of 0.05 to 0.25%. When Sc is added the resultant cast product should be heat treated preferably following the casting operation by holding the cast product at a temperature in a range of 250 to 400xc2x0 C. for a holding time up to 10 hours. The Sc level should not exceed 0.3%, and is preferably in a range of 0.05 to 0.2%.
Further optional alloying element in the alloy according to the invention to improve specific properties can be up to 0.6% Cobalt alone or in combination with up to 0.6% Cerium, and Strontium up to 0.04%.
Ti is important as a grain refiner during solidification of both cast products and welded joint produced using the alloy of the invention. A preferred maximum for Ti addition is 0.2%, and where a more preferred range is of 0.01 to 0.14%.
Fe is a known element in aluminum casting alloys and may be present in a range up to 1.0%. At higher levels Fe may form undesirable large compounds with Mn in the holding furnaces typically employed in casting operations. When higher fracture toughness and/or ductility is desired a suitable maximum for the Fe content is 0.5%, and more preferably 0.3%, and most preferably 0.2%.
Si is a known impurity element in aluminum casting alloys, and normally should not be present a too high levels to avoid the loss in primary strengthening element Mg. However, in the present aluminum casting alloy it can be present is a range of up to 1.4%. Although at higher Si-levels the elongation is somewhat reduced, still very acceptable high levels of elongation in combination with high strength levels are obtained. In a preferred embodiment the Si level should not be more than 1.0%, and more preferably not more than 0.5%, and most preferably not more than 0.3%. A suitable minimum Si-level is 0.10%, and more preferably 0.15%.
Be may be added to AlMg casting alloys to prevent oxidation of the magnesium in the aluminum alloy, the amount added varying with the magnesium content of the alloy. As little as up to 0.005% causes a protective beryllium oxide film to form on the surface. Preferably, the Be level has a maximum of 0.005%, and more preferably is absent without deteriorating the properties of the cast product with this aluminum alloy.
The balance is aluminum and inevitable impurities. Typically each impurity is present at 0.05% maximum and the total of impurities is 0.25% maximum.
In an embodiment of the aluminum casting alloy according to the invention the alloy is capable of achieving in the as-cast condition a yield strength of more than 160 MPa, and in the best examples of more than 175 MPa, in combination with a tensile strength of more than 250 MPa, preferably more than 280 MPa, and in combination with an elongation of more than 10%, and in the best examples even more than 12%. By optimising the casting parameters, further improved tensile properties, and in particular in elongation, can be obtained. Furthermore, improvements in the mechanical properties of the alloy according to the invention can be obtained heat-treating a cast product as is conventional in the art. This further improvement is achieved at the expense of the loss of the earlier advantage that following casting no further heat treatments are required to achieve a desirable level of mechanical properties.
In another embodiment of the aluminum casting alloy according to the invention the alloy is capable of achieving in the as-cast condition a yield strength of more than 120 MPa, and in the best examples of more than 140 MPa, in combination with a tensile strength of more than 210 MPa, preferably more than 240 MPa, and in combination with an elongation of more than 17%, and in the best examples even more than 23%. By optimising the casting parameters, further improved tensile properties, and in particular in elongation, can be obtained. Furthermore, improvements in the mechanical properties of the alloy according to the invention can be obtained heat-treating a cast product as is conventional in the art. This further improvement is achieved at the expense of the loss of the earlier advantage that following casting no further heat treatments are required to achieve a desirable level of mechanical properties.
The aluminum-magnesium casting alloy in accordance with the invention may be processed by various casting techniques. For example, the alloy may be used with resin-bound sand cores and moulds. The best advantages are achieved when applied via permanent mould casting, die-casting, or squeeze casting. In particular when die-casting processes are applied, including vacuum die-casting processes, the best combination of desirable properties and castability characteristics is obtained. It is believed that by applying vacuum die-casting the weldability characteristics of the alloy according to the invention may be further improved. It is to be understood here that die-casting includes high-pressure die-casting operations.
In another aspect of the invention there is provided in a method of producing a cast aluminum product, preferably a die-cast product, comprising the aluminum alloy of the invention as set out above, and casting, preferably die-casting, a body of the aluminum alloy. Following the casting operation the (die-)cast body can be aged at a temperature in the range of 140 to 250xc2x0 C. for a soaking time at this temperature in the range of 0.5 to 24 hours.
The invention will now be explained by reference to non-limiting examples.